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Ag Agency: Commercial Compost Safe

John Dillon
Tom Moreau, manager of the Chittenden Solid Waste District, inspects clover plants in his test greenhouse. The plant in his right hand was grown in a mixture that had trace levels of herbicide.

A year after contaminated compost wilted gardens in northern Vermont, officials are re-assuring the public that commercial compost is safe.

The Agency of Agriculture says chemical herbicides found in trace levels in compost are now being minimized to protect crops.  The agency has released a list of frequently asked questions about the compost issue.

Agency officials say the chemicals that got in the compost were applied legally to feed crops and hay fields both in Vermont and in other states.

Some of the feed crops were fed to horses. And it was horse manure that was the main cause of the contaminated compost sold by the Chittenden Solid Waste District last year. The district halted compost sales and reimbursed several hundred gardeners for their ruined crops.

Jim Leland, director of the agency’s resource management division, said the year-long investigation was frustrating because of false leads and complex science.

“Typically you’re looking for something where you know a use occurred and you’re trying to determine if it’s there and how much,” he said. “In this case you’re unsure as to where and when and how the use occurred and you’re looking at a variety of ingredient streams into the composting facility.”

Agency officials, working with the Chittenden Solid Waste District, identified two chemicals of concern: clopyralid and aminopyralid.

Leland said commercial composters have learned how to live with very low levels of the clopyralid herbicide.

“We believe that there’s enough information to be able to adequately manage the composting process to reduce or eliminate these harmful sources of residues,” he said.

Clopyralid is widespread in the environment and can even be found in food scraps, because it’s used to treat grain fields. Tom Moreau, the manager of the Chittenden Solid Waste District, said he had 25 commercial horse grains tested for the chemical.

“Everyone has come back with substantial amounts of clopyralid,” he said.

Moreau said the challenge for composters is to keep the levels of clopyralid low enough so it doesn’t cause damage.

“With clopyralid it becomes an algebra problem,” he said.

The algebraic formula is used determine the safe amounts of material to blend into the compost recipe.

The other chemical – aminopyralid – is much harder to manage because it causes damage at levels of 1 part per billion or less.

Cary Giguere, the Agriculture Agency’s agrichemical section chief, said the state’s investigation shows the herbicide was used legally.

What we’re seeing was products used on sites that were on the label – so legal uses – that still make it to the compost,” he said.

Aminopyralid was used to treat hay fields, including fields used to grow horse hay in Vermont. Giguere said the herbicide will no longer be used in Vermont because the manufacturer, Dow Agroscience, agreed to put new restrictions on its use. The chemical can no longer be sold in the Northeast for use on pasture and hay fields. Giguere said Dow’s action should greatly reduce the risk of the chemical in Vermont commercial compost.